VLSM and CIDR Malin Bornhager Halmstad University

Transcription

1 VLSM and CIDR Malin Bornhager Halmstad University Session Number 2002, Svenska-CNAP Halmstad University 1

2 Objectives Classless routing VLSM Example of a VLSM calculation 2

3 Classless routing CIDR (Classless InterDomain Routing) Network not identified based on default classes Router uses prefix (/) to describe how many bits are network bits More efficient use of IPv4 address space Aggregation reduces the size of the routing table 3

4 Classless Routing Protocols Includes the subnet mask in the routing updates RIP v2 EIGRP OSPF IS-IS BGP 4

5 CIDR and Route Summarization The first 20 bits in all four networks are common, and can be summarized as a single route 5

6 Calculating Route Summarization Step 1: List the networks in binary format

7 Calculating Route Summarization Step 2: Count the number of left-most matching bits to determine the mask matching bits, corresponds to /20 or

8 Calculating Route Summarization Step 3: Copy the matching bits and add zeros to determine the network address Copy Add zero bits All four networks will be summarized as /20 8

9 Route Summarization 9

10 What is VLSM? VLSM = Variable Length Subnet Mask Different subnet masks within the same network address space Sub-subnetting Used for efficient and scalable addressing 10

11 Subnet mask 11

12 Benefits with VLSM More efficient use of IP addresses Variable size of the networks Greater capability to use route summarization Will reduce the entries in the routing table 12

13 How to calculate VLSM: You have a network that is divided into the subnetwork address /20 This subnetwork is further divided into the VLSM address /26 Calculate the subnet addresses that can be used in the network 13

14 How to calculate VLSM: Use the following steps: Write in binary form Draw a vertical line between bit 20 and 21, where the original subnet boundary was Draw a vertical line between bit 26 and 27, to extend the subnet boundary Calculate the 64 subnet addresses with the bits between the two lines Remember: only unused subnets can be further subnetted 14

15 How to calculate VLSM: 15

16 How to calculate VLSM: Use the following steps: Write in binary form Draw a vertical line between bit 20 and 21, where the original subnet boundary was Draw a vertical line between bit 26 and 27, to extend the subnet boundary Calculate the 64 subnet addresses with the bits between the two lines Remember: only unused subnets can be further subnetted 16

17 How to calculate VLSM: 17

18 How to calculate VLSM: Use the following steps: Write in binary form Draw a vertical line between bit 20 and 21, where the original subnet boundary was Draw a vertical line between bit 26 and 27, to extend the subnet boundary Calculate the 64 subnet addresses with the bits between the two lines Remember: only unused subnets can be further subnetted 18

19 How to calculate VLSM: 19

20 How to calculate VLSM: Use the following steps: Write in binary form Draw a vertical line between bit 20 and 21, where the original subnet boundary was Draw a vertical line between bit 26 and 27, to extend the subnet boundary Calculate the 64 subnet addresses with the bits between the two lines Remember: only unused subnets can be further subnetted 20

21 How to calculate VLSM: 21

22 How to calculate VLSM: Use the following steps: Write in binary form Draw a vertical line between bit 20 and 21, where the original subnet boundary was Draw a vertical line between bit 26 and 27, to extend the subnet boundary Calculate the 64 subnet addresses with the bits between the two lines Remember: only unused subnets can be further subnetted 22

23 Results 23

24 Example: Point-to-Point Links 24

25 Example: Point-to-Point Links Subnet 6 divided to sub-subnets 25

26 Example: Point-to-Point Links 26